Relationships between Indonesian rainfall and Indo-Pacific sea surface temperatures
(SSTs) and circulation anomalies are investigated using observations for 195197.
Indonesia receives significant rainfall year-round but experiences a wet season that
peaks in January and a dry season that peaks in August. Dry season rainfall anomalies
are spatially coherent, strongly correlated with SST, and tightly coupled to El
Niño Southern Oscillation (ENSO) variations in the Pacific basin. Drought
conditions typically occur during El Niño, when SSTs surrounding Indonesia are
cool and the Walker circulation is weakened, resulting in anomalous surface easterlies
across Indonesia. The opposite tends to occur during La Niña. Broadscale
Indonesian rainfall and SST anomalies tend to not persist from the dry season into the
wet season. Rainfall in the heart of the wet season tends to be uncorrelated with SST
and spatially incoherent.

Seasonally varying feedback between Indonesian SST, winds, and rainfall explains the
growth, persistence, and coherence of the local anomalies during the dry season and
their decay or change in sign once the wet season commences. During the dry season
anomalous surface easterlies, remotely driven by warm SSTs in the central Pacific during
El Niño, act to increase local wind speed, cooling the ocean surrounding and to
the east of Indonesia and thereby increasing the anomalous SST gradient across the
Pacific. Hence, local rainfall and the Walker circulation are further reduced. Once the
wet season commences and the climatological surface winds across Indonesia shift from
southeasterly to northwesterly, the anomalous surface easterlies now act to reduce the
wind speed. The initial cold SST anomaly is damped, reducing the negative rainfall
anomalies and surface easterlies. The opposite scenario occurs during La Niña.

Indonesian rainfall variations during the dry season are also coupled to the development
of an anomalous zonal SST gradient in the equatorial Indian Ocean. This anomalous
gradient is strongly related to ENSO and is most prominent during the dry season. Once
the wet season commences, the entire Indian Ocean tends to have the same-signed SST
anomaly (positive during El Niño and negative during La Niña). Development
and decay of this anomalous zonal SST gradient in the Indian Ocean is promoted by
seasonally varying airsea interaction in the eastern Indian Ocean in response to ENSO
conditions in the Pacific. The eastern Indian Ocean SST changes are driven largely by
induced surface heat flux variations (primarily changes in latent heat flux and net
shortwave radiation). Biennial variations in the Indonesian region may also be induced
by this seasonally varying airsea interaction associated with ENSO.